It is known in a conventional boost-buck converter control system that current stagnation, in which a reactor current stagnates or stays around zero (0) volt, occurs when a flow direction of current flowing between a battery and a voltage converter is switched over in a state that a voltage of the battery is stepped up to be supplied to a load as a system voltage in a vehicle. The switchover of the flow direction of current is caused when a load state is switched over between power running and regenerative running of the vehicle.
The following patent documents 1 to 4 disclose conventional boost-buck converter control systems.
[Patent document 1] JP-A-2011-55676
[Patent document 2] JP-A-2004-120844
[Patent document 3] JP-A-2009-148119
[Patent document 4] JP-A-2006-311635
Patent document 1 discloses suppression of variation of a system voltage when current stagnation occurs in a boost-buck converter control system. When a switchover of polarity of a power supply current is estimated by calculating the power supply current based on an accelerator position of a vehicle, electric power of a motor and a battery voltage, a power supply duty ratio is corrected by a value, which corresponds to a predetermined dead time.
Patent document 2 discloses outputting of a correction value for suppressing variation of a system voltage from a correction circuit based on a determination result of a current flow path of a boost-buck converter in a boost-buck converter control system. The current flow path of the boost-buck converter is determined based on a comparison of an inverter output power, which is calculated by an arithmetic calculation circuit, with a predetermined value by a comparator.
Patent document 3 discloses setting of a PWM voltage command value to zero when an absolute value of a difference between a target voltage and a DC bus voltage developed between a boost-buck converter and a load is equal to or larger than a predetermined voltage and a current value of power supplied by the boost-buck converter is equal to or less than a predetermined value. The PWM voltage command value is set to zero by replacing an integration component value included in a duty ratio of the boost-buck converter is substituted with a reciprocal of a proportional component.
Patent document 4 discloses feedback-controlling an output voltage, which is calculated based on a reactor current detected by a current sensor in a boost-buck converter control system, when high response characteristic is needed at the time of rapid changes in a motor power load or a command voltage.
According to patent document 1, the current stagnation is estimated by comparing a detection value of a sensor with a predetermined value and the duty ratio of the boost-buck converter is corrected by a predetermined correction value in accordance with the detection value of the sensor. It is not possible, however, to correct the duty ratio with an appropriate correction value because of manufacturing error of the sensor or variation in a dead time corresponding to manufacturing error of switching elements of a voltage converter. If the correction value is excessively small, the current stagnation will continue and rapid variation of the system voltage will not be suppressed sufficiently. If the correction value is excessively large, the current will vary rapidly and the battery voltage will rapidly vary. That is, it is not possible to control the current smoothly. According to patent documents 2 and 3, the current stagnation cannot be detected accurately either and hence the duty ratios cannot be corrected appropriately. According to patent documents 3 and 4, current sensors are needed additionally.